The invention relates to a process for cleaning the seats of double seat valves and to a control device for applying the process.
A number of control devices of the class characterized above are known [DE 80 03 805 U1, DE 31 08 973 C2, and DE 31 33 273 (2)], all of which embody the same principle of operation. According to this principle, the driving piston of the closing element adjusting rod of the closing element to be lifted within the individual adjustment device is caused to strike a lift limit stop. In this situation the drive piston stop position can be varied within limits by movable stops inside the individual movement device (DE 80 03 805 U1), or again the stop position may be varied relative to the closing element adjusting rod (DE 31 00 973 C2). In the state-of-the-art control device known from DE 80 03 805 U1, the individual adjustment devices are integrated into the primary adjustment device in such a way that one is positioned at the upper and the other at the lower end of the primary adjustment device.
In the control device known from DE 31 08 973 C2, the two closing element adjustment rods are guided upward by the primary adjustment device and end in a control cylinder common to both individual adjustment devices. Lastly, DE 31 33 273 C2 describes a control device in which the individual adjustment devices for both closing elements are introduced additively below the primary adjustment device generating the full stroke for the full open position as an independent adjustment device without modification to the adjacent standard components, between primary adjustment device and spacer housing.
The complexity of the state-of-the-art double seat valves for which seat cleaning may be effected is suggestive of the difficulties that may be encountered in presetting of the stop positions for the drive pistons of the individual adjustment devices. One difficulty is the sum of the manufacturing tolerances of a great number of relevant components necessitating in each instance individual adjustment of the individual strokes, to be added to which is the fact that a check must always be made to determine if the basic operation of the double seat valve is still ensured within the potential partial stroke adjustment range. Specifically, it is necessary in all cases to prevent displacement of the other closing element to a partly open position when one of the closing elements is lifted for the purpose of cleaning the seat of the element. Secondly, the valve seat cleaning processes that can be carried out with state-of-the-art control devices exhibit a fundamental disadvantage resulting from stationary retention of a partly open position of a particular closing element, once this position has been reached, until the valve seat cleaning process has been ended by a control signal fed externally to the double seat valve. Since the drive piston of the individual adjustment device is caused to strike a stop position, as has already been pointed out, a more or less large passage cross section is available for the cleaning agent over the entire duration of the particular seat cleaning process. The pressure and temperature conditions to which double seat valves of the class indicated in the foregoing are subjected in practical applications may either reduce or enlarge the passage cross sections that have been produced. A reduction may cause inadequate seat cleaning, while in all cases enlargement results in higher cleaning agent consumption, without the cleaning effect necessarily being increased as a result. Especially in the event of a large number of double seat valves to be cleaned simultaneously, as for example within a valve block, this latter aspect necessitates generation of a too high and thus cost ineffective cleaning output, without achievement at least of intensification of the seat cleaning process or seat cleaning of equal intensity for all double seat valves.
In connection with a state-of-the-art process for realization of a leakage free connecting double seat valve for which seat cleaning is possible (EP 0 208 126 B1), in which a closing element designed as a slide displaced in the direction of a valve housing element, such as one guiding a product, for the purpose of cleaning the valve seat, the other closing element, designed as a seat plate, being displaced simultaneously relative to the element, such as one acted upon by a product, by the amount of a partial stroke in the same or opposite direction, it has admittedly been proposed that the seat cleaning process be effected by repeated partial stroke movements of the closing elements. The purpose of this measure was primarily to free the seat surface of the closing element designed as a slide and in the process to apply a film of cleaning agent between the seal seat and seat surface as well, by means of repeated wiping. The critical area between seat seal and seat surface of the closing element, such as one acted upon by a product, could not be cleaned reliably exclusively by a single opening stroke. But even in this state-of-the-art process, the drive pistons of the individual adjustment devices are caused to strike against a fixed stop position, which is retained until a control signal supplied from outside the double seat valve ends the lifted position over the duration of which the entire passage cross section is available for cleaning agent flow.